1. Field of the Invention
The present invention relates to test of a video signal, and more particularly, to a method and apparatus for performing a function test of a video signal generator.
2. Related Art
As a video display becomes a high resolution with high fidelity of color, the quality of a video signal generator becomes very important. For this reason, it is required strongly to perform the function test of the video signal generator accurately and at high speed.
An example of a conventional test apparatus is disclosed in Japanese Patent Laid Open No. Sho 60-229092. In FIG. 1, a video signal generator 900 is a circuit to be tested, and the other circuits constitute the conventional test apparatus. The same circuit as the video signal generator 900 is incorporated in the test apparatus. More particularly, the test apparatus has a dot counter 903, a display controller 905, a multiplexer 906, an image memory 908, a blink counter 909, and a display control circuit 910. These circuits correspond to the circuits 912-916 incorporated in the video signal generator 900.
The test apparatus extracts a dot clock signal 919, a vertical sync signal 920, and a blink signal 921 from the video signal generator 900 through probes and supplies them to a character clock comparator 902, a vertical sync signal comparator 904, and a blink signal comparator 907, respectively. These signal are subjected to three steps of synchronization to synchronize the test apparatus with the video signal generator 900 such that the operation states of both of the test apparatus and the video signal generator 900 is completely identical. After the synchronization has been completed, a video signal 923 output from the display control circuit 910 is compared with a video signal 922 output from the display control circuit 917 by a video signal comparator 918 using the dot clock signal 919 as a sampling clock. Using the comparison results, a central processing unit 901 determines whether or not the video signals 923 and 922 are identical to each other. When either one of the dot clock signal 919, the vertical sync signal 920 or the blink signal 921 cannot be synchronized during the determination of the video signals, the character clock comparator 902, the vertical sync signal comparator 904 or the blink signal comparator 907 generates an interrupt signal to the central processing unit 901. The interrupt signal inhibits the central processing unit 901 from determining the output of the video signal comparator 918 until the test apparatus and the video signal generator 900 are synchronized with each other.
However, the conventional test apparatus is necessary to be provided with a circuit having the same function as the video signal generator 900 to be tested. Furthermore, the dot clock signal 919 is extracted from the oscillator 911 of the video signal generator 900 and is used for the character clock comparison and the sampling of the video signals. Therefore, it is required to provide different test apparatuses depending on the function and operation speed of the video signal generator 900 to be tested.
In addition the circuits having the same function as the video signal generator 900, the respective comparing circuits of the character clock signals, the vertical sync signals, the blink signals and the video signals are required. Therefore, the circuit structure becomes complicated and the amount of hardware increases. Furthermore, as the number of dots in the color display is increases, an image memory having a larger amount of capacity is required in the test apparatus.
Further, in the conventional test method, since various types of test are performed after the synchronization with the video signal generator has been established, the sync signals is not tested quantitatively. For this reason, every time the test apparatus is out of synchronization with the video signal generator, the test has to go back to the beginning. Furthermore, since it is unknown whether the sync signals are generated by the video signal generator precisely or not, it is difficult to specify a failed function of the video signal generator accurately.